This explanation of the special and general theories of relativity combines Albert Einstein's original text, written for the layperson in 1916, with the insights of nearly a century of subsequent developments in physics. In simple terms, Einstein lays out the basic concepts of relativity, then physicist Robert Geroch, one of the world's preeminent experts on relativity, builds on Einstein's work with commentary that addresses the ideas at the heart of the theory. Mathematician Roger Penrose's introduction frames the creation of both the special and the general theories in the history of science, arguing that the general theory is so unique and revolutionary that, without Einstein, we still may not have it today. And science historian David Cassidy shows that while the theory reinforced the idea of scientific progress for some, the counterintuitive nature of the theory alienated many nonscientists, opening a rift between science and the public that continues to grow today.